1. Field of the Invention
The present invention relates to a color display element and a method for driving the same.
2. Description of the Prior Art
In arranging a reflecting-type liquid crystal display which eliminates the use of a backlight, in order to provide a natural appearance, it is an essential requirement that a polarizing plate not be used and that only the natural light incident into a liquid crystal element from the outside be used. In order to achieve such a display, it is desirable to use a guest host type or light scattering type display system. Among these, a reflective type liquid crystal panel using a dimmer of the above-described guest host type is arranged as shown in FIG. 1A. That is, in a color liquid crystal panel of this system, a color filter 2 is provided on the surface of one transparent substrate 1 to determine the spectrum of transmitted light, and a transparent electrode 3 is formed on the color filter 2. On the other substrate 7, a transparent electrode 5 and a light reflecting layer 6 are laminated. A liquid crystal is encapsulated within the gap formed between transparent electrodes 3 and 5 to form a dimmer layer 4. Not only are transparent electrode 5 and light reflecting layer 6 are separately formed, but also the former may also serve as a light reflecting layer to form a metal reflecting mirror. As a liquid crystal display device having such an arrangement, one using a black dyestuff guest host type liquid crystal as the dimmer layer 4 is disclosed in Eurodisplay '87 speech No. P2.4. In this arrangement, incident light passing through the transparent substrate 1 passes through color filter 2, transparent electrode 3, dimmer layer 4 and transparent substrate 5 to be reflected by light reflecting layer 6, which reflected light is then emitted from transparent substrate 1 to the outside along the path opposite the path described above. If light which has followed such a path has passed through a red color filter, it will appear red when seen from the outside because it is emitted from transparent substrate 1 to the outside with a red spectrum. Therefore, a display having this arrangement allows a color display.
Another reflecting color display with the arrangement as illustrated in FIG. 1B employs transparent electrode 3 which is formed on the surface of transparent substrate 1. On another substrate 7, transparent electrode 5, color filter 2 to determine the spectrum of the transmitted light, and light reflecting layer 6 are laminated. A liquid crystal is encapsulated into the gap between these transparent electrodes 3 and 5 to form dimmer layer 4. Incidentally, the order in which transparent electrode 5 and light reflecting layer 6 are disposed is not necessarily restricted to the one illustrated in FIG. 1B, and there are also cases in which their functions are integrated. An example of a liquid crystal color display having such an arrangement in which dimmer layer 4 is arranged by using a black dyestuff guest host polymer dispersed liquid crystal (PDLC) is described in the January 1991 issue of the journal DISPLAY on page 2. In this liquid crystal color display, incident light which passes through transparent substrate 1 passes through transparent electrode 3, dimmer layer 4, transparent electrode 5, and color filter 2 to be reflected against light reflecting layer 6, following which the reflected light travels along the path opposite the above path to be emitted from transparent substrate 1 to the outside. If passed through a red filter, this light will appear red when seen from the outside because it is emitted from transparent substrate 1 to the outside with a red spectrum. Therefore, a display element having this arrangement also allows a color display.
There is an additional example of a reflective monochrome display shown in FIG. 1C in which transparent electrode 3 is formed on the surface of transparent substrate 1, and transparent electrode 5 and light absorbing layer 8 are laminated on the other substrate 7. Liquid crystal is sandwiched between these transparent electrodes 3 and 5 to form dimmer layer 4. A display having a dimmer layer using polymer dispersed liquid crystal (PDLC) is disclosed in 13th International Liquid Crystal Conference, speech number APP-34P-P-Tue. In this arrangement, incident light passing through transparent substrate 1 passes through transparent electrode 3, and, if dimmer layer 4 is transparent, continues through dimmer layer 4 and transparent electrode 5 to be absorbed by a light-absorbing layer 8 with the result that a black display is obtained. If dimmer layer 4 scatters light, then the incident light passing through transparent substrate 1 passes through transparent electrode 3 to enter dimmer layer 4. Since dimmer layer 4 is light-scattering, the incident light is then scattered with some of the light passing back through transparent electrode 3 and transparent substrate 5 to be emitted to the outside. The remainder of the scattered light passes through transparent electrode 5 to be absorbed by light-absorbing layer 8 and is not reflected. As a result, when seen from the outside, the light appears white and a white display is obtained. As described above, the display illustrated in FIG. 1C enables a monochrome display depending on whether dimmer layer 4 is transparent or light-scattering.
A conventional reflective liquid crystal color display has an arrangement in which color filter 2 is disposed in front of dimmer layer 4 (FIG. 1A) or behind dimmer layer 4 (FIG. 1B). Therefore, in the displays illustrated in FIGS. 1A and 1B, since dimmer layer 4 changes between black state and transparent state, if black display and color display are desired, there will be no major problem regarding the quality of the display. However, if white display is desired, since the light incident into the color display all passes through the color filter and at least two thirds of that light energy is lost, a perfectly white display will not be obtained. A gray display is considered sufficient for monochrome display. If dimmer layer 4 is switched between the transparent state and the light-scattering state as shown in FIGS. 1A and 1B, it is possible to achieve a color display when dimmer layer 4 is transparent, but if dimmer layer 4 is in the light-scattering state as in FIG. 1A, the incident light must pass through color filter 2, resulting in a gray display which achieves neither a white or black display. In the case of FIG. 1B, the scattered light is emitted from transparent substrate 1 to the outside, as it is, resulting in a white display, in which case, a black display cannot be achieved. Further, in the display of FIG. 1C, while it is possible to achieve a monochrome display, a color display cannot be achieved.